Multiprocessor computers implement varying levels of symmetry. Master-slave processor computer systems are very asymmetric, whereas in computers designed with higher levels of symmetry, each of the working processors are capable of performing the same functions. In symmetric computers, the working processors share buses, address the same memory and basic input/output system (BIOS) resources, and receive the same array of interrupts.
Symmetric multiprocessor computers also tend to have a non-hierarchal bus arbitration process. The Intel.RTM. Pentium-Pro.RTM. Processor Bus arbitration protocol, for example, supports fair arbitration between a maximum of four working processors in the typical multiprocessor configuration. A unique identification, i.e., agent ID of 0, 1, 2, or 3, is assigned to each working processor as part of the power-on configuration by a central agent that is also responsible for bus arbitration control. Bus arbitration between the working processors is then made using a round-robin algorithm, which arranges the working processors in a circular order of priority: 0, 1, 2, 3, 0, 1, 2, etc. The working processor with the highest priority becomes the bus owner on every arbitration event, and the working processor that owns the bus is allowed to request its use. Input/output and possibly memory subsystems may, however, have higher priority, defeating the immediate use of the bus by the working processor.
The symmetry in some multiprocessor computers tends to break down when considering reset protocol in which the computer is bootstrapped after being powered-on or rebooted after an operating system failure. Pentium-Pro.RTM. multiprocessor systems fall into this class. The same working processor (called the boot strap processor) performs the bootstrapping operations every time the computer is powered-on. The bootstrapping can generally not be assigned among the other working processors even though they have access to the same computer system resources and would thus be capable of performing the role of boot strap processor.